The invention relates to a method and corresponding devices for the continuous mechanical dewatering of aqueous suspensions or elutriations, particularly waste paper suspensions or slurries, between an endless mesh band and an endless compression surface exposing one closed, smooth surface and moving in the direction of operation. With this arrangement, the suspension cake to be dewatered is compressed between the mesh band and the compression surface. The requisite pressing power is achieved by wrapping the mesh band around the cylindrical compression surface under longitudinal tension during the dewatering process, whereby the water that is pressed out is removed from the suspension cake by use of the mesh band.
This thickener is also suitable for integration into existing dewatering processes with the goal of increasing the consistency of solid matter, without additionally increasing substantially the building space or operational complexity.
One of the heretofore known processes, e.g. the xe2x80x9cAngle Pressxe2x80x9d of Bellmer from 1972, drives the suspension cake between two mesh bands; this has the advantage that the cake can be dewatered on two sides, while this sandwich is led in a meandering fashion around guiding rollers, resulting in the removal of expelled water from the roller through the mesh band looped around it, which is in the meantime outwardly disposed. The diameter of the guiding roller can also be reduced in the course of the loops, which increases the pressure. However, it proves to be an aggravating disadvantage that the water that remains in the meshes of both mesh bands due to adhesive force flows back into the cake after leaving the compression region, because the adhesive force of the suspension due to the finer fiber and particle sizes is stronger, and thus causes reverse suction.
However, the guiding roller diameters cannot be arbitrarily reduced, because the strength and deflection of the guiding rollers prohibits this.
Another possibility for increasing the compression lies in raising the longitudinal tension of the mesh band; here also the strength and deflection of the guiding rollers, as well as the strength of the mesh band itself, place limits on the increase of the solid matter proportion after the pressing.
In practice, the mesh band tensions are on the order of magnitude of 10 N/mr, seldom up to 30 N/mm, which, with a small roller diameter of, e.g., 200 mm, results in a pressure of about 1 bar, seldom 2 bars.
However, neither possibility prevents the rewetting due to the water in the mesh.
A second process uses only a single mesh band (Swiss patent CH 644414, xe2x80x9cVariosplitxe2x80x9d), which in part surrounds a smooth roller. The advantage lies in the rewetting of the cake on only one side and the simpler implementation with just one band. However it is disadvantageous that the cake can be dewatered only on this one roller, i.e. only a single time and only on one side, so that the resulting dryness proportion is not optimal. Moreover, the dewatered cake thickness is limited also because the water on the side by the roller must take a path through the entire cake thickness to the mesh. This can be somewhat compensated for, however, by a slower operation of the press.
An optimal result is thus not possible: final dryness proportion and mass flow throughput conflict with one another.
The chamber filter press is used as a third process, which uses not a continuous but a batch-wise operation; the involved filling and emptying procedure conflicts with the lengthy duration of operation and the high pressure. In addition, centrifuges (decanters) are set up for the removal of water. These two processes have nothing to do with this patent idea.
Although not belonging to the problem area of this invention, still for the sake of completeness the pressing of water out of, e.g., paper or cardboard sheets, is mentioned:
Fundamental differences in the subject matter of this invention are found in that, with paper, the quality of the sheet to be dewatered must be considered; i.e. that the incoming sheet is very homogeneous, that the pressing is carried out with felts that are insensitive to marking, that no crushing of the sheet can take place, i.e. displacement of the fibers, that the water that is pressed out may not move against the running direction of the paper sheet, because otherwise the paper sheet will be destroyed, and that all in all the quality, i.e. the homogeneity of the dewatered paper sheet, must be guaranteed.
Paper machine felts, which with their fine hair fleece provide for mark-free homogeneous pressing, are not used in the present patent idea. On the one hand, they can cause dirtiness much more easily, and on the other hand, uniform mark-free pressing is not at all necessary with the current invention; on the contrary, a marked, i.e. nonhomogeneous pressing is desired, because this will generate a certain kneading action, which helps a better water flow out of the cake. Moreover, with the current invention it is not desired that the cake form a stable sheet after the thickening. With the idea of the patent, a destruction of the sheet structure is inter alia intended through the intentional differential speed between the mesh band and the compression surface on the two sides of the cake and/or through the coarse structure of the additional water-permeable bands, in order to clear paths for the water during the squeezing out procedure for easier outflow, and in order to achieve a crumbling after the thickening.
In addition, suspensions with paper and cardboard sheets are not brought from a fluid phase to a consistency of over about 25% on the same mesh band, which is accomplished with this invention.
A goal of the invention is to avoid the disadvantages of the techniques that have been employed until now, in particular to combine the advantages of known apparatus, i.e. to increase the effective pressures on the suspension cake clearly and long enough with minimal rehydration, and indeed in a continuous throughput operation.
This problem is solved by the invention by providing fittings that both increase the effective duration of the pressure (dewatering pressure) of the existing mesh band on the suspension cake, and at the same time increase the pressure considerably, without obstructing the outflow of the expelled water. The method according to the invention increases the pressure from, until now, about 1 bar to a pressure of, e.g., up to 100 bars.
Economically interesting solutions result from this development:
By thickening waste paper suspensions, the invention can be directly attached to the washing process, i.e. the separation of fine materials from reusable fiber material by means of dewatering a thin-layer low-consistency suspension ( less than 2%) with a higher working speed ( greater than 200 m/min.), whereby the mesh band used for this purpose is put to use with the guiding rollers for both steps. The transport of the partially dewatered suspension also goes from the washer to the thickener.
The thickening of slurries by means of the double-mesh press will lead to a lower water content of the mechanically dewatered cake, through the integration of the invention in the known process (ingoing consistency of  less than 2% with an operating speed  less than 10 m/min.). This is of high general economic benefit: high costs are incurred these days for the removal of the thickened slurry and for the then-necessary disposal space, drying or burning, respectively, since ⅔ to xc2xe of the volume consists of water. Reducing the water content has a direct positive effect on these costs.
Advantageous arrangements of the patent idea should improve the efficiency and range of application. The application to not very uniformly distributed suspension cakes and the avoidance of rehydration during the separation of the already thickened cake and the first mesh band are given particular attention.
The claims specify that the pressure is increased by running several continuous bands on top of one another. The pressure is thereby correspondingly increased, though the band tensions remain in customary ranges.
The claims deal with different band constructions, which make possible the flowing through of expelled water. Thus, in the claim a band is claimed that holds no water in its holes, and accordingly also cannot allow return flow of such water.
The claims deal with the high inherent stiffness of the band, in order to bridge over unevennesses of the cake or to even out the pressure, which both rolls the cake and aids the outflow of the expelled water.
By the use of a compression shoe with hydrostatic or hydrodynamic lubrication, the sliding characteristics between the band and the sliding surface are likewise improved. The inherent stiffness of the band can be very high in the direction transverse to that in which the machine runs, as long as the longitudinal stiffness remains so low that the band can easily follow the contours of the rollers and the sliding surfaces.
The claims highlight alternatives wherein the expelled water can flow, in particular can be transported out of the compression region, without the pressing out process being hindered due to mangled amounts of material that could be sucked in.
The claim shows that the still unstable filter cake is subjected to increasing pressure, while several bands are run through after one another and, only as the last step, the high pressure of the compression shoe takes place.
The claims are directed to the prevention of the reverse flow of free water from the bands into the cakes by separating them from one another after exerting the maximum pressure, because this would deteriorate the achieved consistency of the cake. The claim operates on the premise that, given a rapid complete separation of all water-carrying bands (above all using a high operating speed), little time will remain for the water to be sucked back in.
The claim, intended primarily for slow operation, operates on the premise that the first mesh band actually remains on the cake a short while longer and its water can be returned to the cake, but further water-bearing bands take their water with them and do not rehydrate the band.
This is particularly assured when, as in the claim, the band that lies against the first mesh band carries scarcely any water with it. This effect becomes clear from the arrangement of the separation of the bands underneath the cake, where gravity prevents the reverse flow to the first mesh band.
If the rehydration due to the first mesh band itself is to be minimized, there are two possibilities:
One can select therefor a mesh web with small receiving volumes for the pressed water, and in particular this mesh is compressible under pressure, so that only a small amount of water sticks to it after leaving the pressure zone.
Or, according to the claim, one removes the water from the mesh after it leaves the pressure zone and before the mesh is lifted off of the suspension cake. This can be accomplished by tangentially spraying a flat stream of water onto the mesh, whereby the higher stream speed, with its kinetic energy, rips the water from the web mesh. The same thing can happen by blowing of a stream of air. However, these two methods are not very convenient, because the spraying water must be recovered. Therefore it is more sensible to use a suction pipe, which by means of a longitudinal member forms a gap with respect to the mesh, through which the suction air flows. In this case, the water is ripped out of the mesh, and is carried off with the suction air through the suction pipe.
The claims show variations in how the compression surface, against which the first mesh band compresses the suspension cake, can be designed. The claim shows a shoe drum with a radius of curvature that becomes progressively smaller, which is possible without differential speed or friction between the roller cylinder and the mesh band.
The claims involve a modified principle, wherein the compression surface in the pressure zone takes on a concave form (shoe cylinder) and presses against a round cylinder.
Although the fundamental principle of this invention is not necessarily bound to the fundamental spatial placement of the apparatus elements, in the embodiments of the claims; it is of great benefit if the round opposing cylinder is positioned underneath the pressure zone. In this case, the removal of the expelled water is easiest. This is particularly advantageous in the case of the lower speed of operation during the dewatering of the slurry, as in the claim. In addition, press-water can also run off against the rotational direction. It is best to try to lead the bands away after the pressing in a declining slope, so that no mesh water can flow back to the cake.
The claim deals with the possibility that the first mesh band is operated with a different speed relative to the compression surface, whereby the suspension cake is dislocated and torn open, which increases the overall surface and improves the outflow of water.
The inventive idea will be more specifically explained in the following, with the help of simplified and schematically represented example implementations, with reference to the drawings: